Transparent window pane provided with a resistive heating coating

ABSTRACT

A transparent glazing unit including a resistive heating coating that extends over a substantial part of the glazing unit, in particular over a main viewing field, and is electrically connected to at least two busbars such that, when an electrical supply voltage is applied between the busbars, a current flows, which heats a heating field in the coating. The heating field includes at least one semiresistive region in direct contact with at least one busbar.

The invention relates to a transparent glazing unit with a resistiveheating coating that has the characteristics of the preamble of claim 1.

The invention relates more particularly to a glazing unit whoseresistive heating coating is a coating placed on a substrate and havingthermal insulation and/or solar protection capabilities. The glazingunits incorporating this type of coating, when they are intended forequipping vehicles, make it possible in particular to reduce theair-conditioning load and/or reduce excessive overheating (“solarcontrol” glazing) and/or reduce the amount of energy dissipated to theoutside (“low-e” or “low-emissivity” glazing) brought about by the evergrowing use of glazed surfaces in vehicle passenger compartments.

One type of multilayer known for giving substrates such propertiesconsists of at least two metal layers, such as a silver-based layer,each being placed between two coatings made of a dielectric. Thismultilayer is generally obtained by a succession of depositionoperations carried out using a vacuum technique, such as cathodesputtering, optionally magnetically enhanced or magnetron cathodesputtering. Two very thin metal layers, called “barrier layers” may alsobe provided, these being placed beneath, on, or to each side of eachsilver layer, the underlayer as a tie, nucleation and/or protectionlayer, for protection during an optional heat treatment subsequent todeposition, and the overlayer as a protective or “sacrificial” layer soas to prevent impairment of the silver if the oxide layer that surmountsit is deposited by sputtering in the presence of oxygen and/or if themultilayer undergoes a heat treatment subsequent to deposition.

In particular for vehicle windshields, there is high demand in themarket for heating versions, in which the heating means must by naturebe the least visible, or least obstructing for viewing, as possible.Consequently, there is an increasing demand for a transparent heatingcoating for such glazing.

A general problem with heating coatings having a low light absorption istheir relatively high surface resistance, which requires a high supplyvoltage, in any case for heated glazing of large dimensions or for longcurrent paths, which voltage is in any case higher than the usualvoltages on board vehicles. If it is desired to lower the surfaceresistance, this is accompanied, in the multilayer systems knownhitherto, with a reduction in visible light transmission because theconducting layers have to be thicker.

For these technical reasons, glazing units heated by wires, which may besupplied without any problem by the on board voltage, are stillpreferably being mounted at the present time. These laminated glazingunits with integrated heating fields in the form of very fine wires are,however, not accepted by all purchasers.

Patent DE 1 256 812 B1 discloses a glass pane that can be heated by acoating made of metal or a metal oxide deposited continuously on one ofits surfaces. This publication aims to solve problems due to the highohmic resistance of said coating, which is of the order of 200Ω/□.However, to be able to heat this coating using a relatively low voltagefrom two lateral busbars, narrow printed electrodes of low ohmicresistance (called “auxiliary electrodes”) that extend from said busbarsover the heating field are provided. Said auxiliary electrodes terminateonly a short distance in front of the opposite busbar, and they overlapeach other with an alternating polarity.

However, said lines, which are optically perceivable as a hatching,obstruct vision and detract from the optical appearance of the mainviewing field of the pane thus produced. It is not possible to make useof the optical advantage of a transparent heating coating. It is forthis reason that such a pane is designed only for a rear window ofautomobiles.

Another problem with heating coatings may arise owing to the fact thatthey are sometimes not able to be deposited uniformly over the entiresurface of the transparent glazing, but one or more interruptions,called “communication windows”, have to be provided therein, whichdisturb the flow of the heating current and may form “hot spots” (localoverheating) along the edges of this or these communication windows.Such communication windows serve to make the coating, which by nature isreflective for short-wave radiation, respectively infrared radiation,locally more permeable to certain data streams or signals.

To inject and extract the heating current in these coatings, at leastone pair of electrodes (in the form of bands) or of busbars, which haveto inject the current into the heating coating and distribute it over awide front as uniformly as possible, is provided. In vehicle glazing,which is substantially more wide than high, the busbars are usuallyfound along the longer edges of the glazing (in the mounted position,the upper and lower edges), so that the heating current can travel alongthe shortest path over the height of the glazing. At the same time, theaforementioned communication windows are most of the time located at theupper edge of the glazing and extend there over several centimeters ofwidth.

Document WO 00/72635 A1 discloses a transparent substrate with a coatingthat reflects IR rays and a communication window produced locally byremoval or omission of the coating.

Obviously, each communication window which modifies the uniformity ofthe coating disturbs the current flow. Local temperature spots (“hotspots”) appear, which may result in damage to the substrate (thermalstresses) and to the coating itself. This is not only the case when thecoating is defective over a large area, but also when the communicationwindow is formed by a relatively large number of slots that do notcommunicate with one another. These also result, in the surface part inquestion, in an appreciable increase in the layer resistance and alsogive rise to the abovementioned hot spots.

The last document mentioned proposes, as a means of reducing theproblematic effect of an extensive communication window, to provide,along its edge, an electrically conducting band that has an ohmicresistance per square which is significantly lower than that of theheating layer. Said band purports to take the current around the cut.Preferably, a communication window is framed entirely by such a band.The band may be produced by printing a silver-containing conductivescreen-printing paste and by baking it. However, it may also be appliedby the deposition of an electrically conducting lacquer or by depositinga metal strip. In all cases, a conducting electrical connection of theband to the coating is of course necessary in order for it to operate.

The band may be concealed from view by superposing an electricallynonconducting opaque masking strip, for example made of black enamel. Asa general rule, such masking strips are made up from black-colorednonconducting material (screen-printing paste) to be baked. Infraredradiation is not reflected by this material, but absorbed.

Document WO 03/024155 A2 discloses transparent glazing of this type witha heating coating, in which, on the one hand, a maximum nominal voltageof 42 V is indicated, which however aims also to solve the problem of“hot spots” along the edges of a communication window. In general,various voltage levels are used, a lower voltage being applied toshortened current paths (for example because of the communicationwindow) so as to avoid local overheating. Specifically, thecommunication window region is cut out from the heating surface byplacing a separate busbar between the communication window and thebusbar located on the opposite side.

Also known, from document DE 36 44 297 A1, are many examples of heatingcoatings for a vehicle windshield that are divided. The divisions maythus be produced by parts that are not provided with surface layersand/or by notches produced mechanically or by a laser beam. They areused for suitably adjusting and deflecting a current flow within thecoated surface and have to ensure as uniform as possible a currentdensity in the surfaces in question.

Document WO 2004/032569 A2 discloses another configuration oftransparent glazing with a heating coating, which also aims to achieveuniformity of the heating power in the surface by separating linestraced in the coating.

Document DE 29 36 398 A1 relates to measures intended for preventingcurrent spikes in the transition between the busbars and the coating, intransparent glazing with a heating coating. In general, the aim is toreduce the sudden difference in resistance between the coating and thebusbars using materials or shapes with a higher resistance for thelatter, or else with intermediate resistances. The above documentindicates surface resistances of the coating of between 1 and 10 ohmsper unit area. In one of the many embodiments described in thatdocument, the edge of each busbar turn toward the opposite busbar is ofcorrugated form. The formation of sharp points turned toward the heatingcoating must thus be avoided. This approach aims to appreciably lengthenthe transition line between the busbar and the coating and consequentlyto reduce the current density in this transition. However, all thesemeasures seem poorly suited to be able to supply the heating layer witha relatively low voltage.

It is also known to provide, on the incident face of photovoltaic solarcells, grid or comb electrodes (see for example document WO 03/075351A1). They are often produced by screen printing and made up of a busbarplaced along the edge of the solar cell and of a plurality of small combteeth that extend from the busbar over the surface of the solar cell.These electrodes allow surface connection for the photovoltaic voltage,which is present on both faces of the absorber, between the front combelectrode and the rear metal electrode, respectively, over its entiresurface without excessively reducing the penetration of light into theabsorber.

Document DE 197 02 448 A1 discloses a heated mirror, on the glasssubstrate of which two comb-shaped conducting tracks or electrodes areplaced, these being indented one in the other, with a PTC coating (i.e.one having a positive temperature coefficient of resistance) that coversthem and fills the intermediate spaces between the comb teeth. However,that document does not consider the problem of making the heatinginvisible to the eye, because the conducting tracks and the heatinglayer may be placed behind the mirror layer.

Document DE 198 32 228 A1 discloses vehicle glazing with an electricallyconducting coating that is optically transparent and used as an antenna.Purely capacitive high-frequency radio signals are picked up from theantenna layer using a coupling electrode, which is made up of severalfine wires connected together and placed parallel to one another at acertain distance apart that is large compared with their diameter, whichwires extend from the edge into the viewing field of the glazing andterminate therein, without continuing. There is no galvanic couplingbetween the coating and these wires, because each time they lie indifferent planes from the laminated glazing.

The busbars already mentioned many times may be produced on the glasspane equally well by printing (screen printing) before or afterdeposition of the coating, or by soldering thin strips of sheet metal,preferably made of tinned copper. Combinations of printed busbars andmetal-strip busbars are also known (see for example document DE 198 29151 C1). Admittedly, the busbars are usually narrow and in the form ofstrips, but they are not transparent. For optical reasons, they aretherefore placed each time near the outer edge of the transparentglazing units in question. Most of the time, they may be masked byopaque edge coatings (usually produced by screen printing). Likewise,the aforementioned communication windows may be masked by these edgecoatings, provided that they are sufficiently permeable to the radiationto be transmitted via the communication window.

In standard vehicle windshields, these opaque coatings are in the formof a frame provided all around the glazing, which frame also has thefunction of protecting the bonded joint between the glazing and the bodyfrom UV radiation. These frames surround the general viewing field ofthe glazing. In windshields, a distinction may also be made between themain viewing field A, approximately in the middle of the area of theglazing, in which there can be no perceptible impairment of vision (forexample by colorations or wires or other damage larger in size than 30microns), and the secondary viewing field B closer to the edges.

The problem at the basis of the invention therefore consists in how toprovide a transparent glazing unit provided with a heating coating thatcan operate with relatively low nominal voltages, in particular around12 to 14 volts, and which nevertheless produces a uniform distributionof the heating, in particular without any hot spot, with viewing in thegeneral viewing field of the glazing, and in particular in the mainviewing field A of the glazing, which is impeded as little as possible.

This problem is solved according to the invention by the features ofclaim 1. The features of the dependent claims provide preferredembodiments of this invention.

According to the invention, the heating field, formed by the currentflowing between the busbars when an electrical supply voltage is appliedbetween these busbars, includes at least one semiresistive region indirect contact with at least one busbar.

The term “conducting” should be understood within the context of thepresent invention to mean that the element thus termed admittedly has anelectrical resistance, as it is out of the question here to usesuperconductors, but that this resistance is very low, in such a waythat when the electric current used flows through this element, it doesnot heat up so as to be perceptible by touching it with one's handwithin one minute of applying voltage, that is to say that this elementwill be classed as constituting a cold region when the glazing unit isobserved by thermography.

The term “resistive” should be understood within the context of thepresent invention to mean that the element thus termed has a highoverall electrical resistance, in such a way that, when the electriccurrent used passes through this element, it heats up so as to beperceptible by touching it with one's hand within one minute afterapplying voltage, that is to say that this element will be classed asconstituting a hot region when the glazing unit is observed bythermography. Within the technical field in question, the resistiveregions have a surface resistance of around 0.5 to 5 ohms per unit areaand the hot regions formed at their places have power densities of atleast 400 to 450 watts/m².

The term “semiresistive” should be understood within the context of thepresent invention to mean that the element thus termed has a low overallelectrical resistance, which is less than that of the resistiveelement(s), but greater than that of the conducting element (s). Theexpression “semiresistive region” used in particular here denotes anarea having a low overall resistance. However, the region may have ahigh resistance in certain places and a very low resistance in otherplaces. For example, it may even include conducting elements andresistive elements, the combination and configuration thereof make saidregion “semiresistive”.

The heating field is the direct result of the electric field whenvoltage is applied to the terminals of the electric field. It alsodenotes the actual heating region of said glazing, which extends betweenthe two busbars.

The object of the present invention is to create a novel heating fieldthrough the use of a particular electric field. As in the electricfields of the prior art, the ends of the electric field are formed byconducting regions embodied by the busbars, and between these busbars anelectric field is produced. However, unlike the prior art, the surfaceresistance of this field is not uniform over its entire area—asemiconducting region is produced in contact with at least one busbar.This has the effect of promoting electrical conduction in this regionthrough which the current flows and has the effect as it were offavoring energy transport into the following resistive region.

At least one part of the electric field according to the invention thushas the following scheme, from one busbar to the other:

-   -   conducting region/semiconducting region/resistive region/ . . .        /conducting region.

Thus over at least one path between two busbars, the current willfirstly pass through a semiconducting region and then through aresistive region.

The electric field between the busbars thus has a resistivity gradientformed from at least two separate (semiresistive/resistive) states.There may also be a gradual gradient formed from many states, thuspassing gradually from the conducting state to the resistive state andthen returning to the conducting state.

This may all be observed by thermography.

The present invention is of most particular importance in the “panoramicwindshield” technology.

In this technology, the aim is to produce windshields that are as wideand/or as tall as possible, which include portions extending laterallyalong the sides of the vehicle and/or on the roof of the vehicle.

Thanks to the invention, it is thus possible to produce heatingpanoramic windshields in which the heating power is concentrated in theessential part of the glazing, namely the main viewing field A.

With these features and arrangements, a relative shortening of the pathof the current flow within the coating of relatively high resistanceitself is obtained because part of the distance between the actualbusbars and the main central heating field, in the main viewing field A,is crossed by low-resistance auxiliary conductors or else by saidsemiresistive regions.

The main viewing field A of the glazing preferably has no semiresistiveregion and thus it remains optically free of any perturbations orobstructions.

In an alternative embodiment, at least one semiresistive region ispreferably in direct contact with at least one busbar at the positivepotential.

At least one other semiresistive region is then preferably in directcontact with at least one busbar at the negative potential and the mainviewing field A then preferably lies between said two at leastsemiresistive regions.

In one particular version of the invention, the semiresistive regionincludes conducting strands formed from conducting printed lines,preferably printed on the heating coating 2 and/or from conductingwires, these conducting wires preferably being electrically connected tothe heating coating and at least to said semiresistive region bysoldering at least at discrete contact points.

The conducting strands cover only part of the heating field (close tothe edges of the glazing), especially a relatively wide strip along thebusbars.

They terminate blindly, preferably before the boundary of the centralviewing field A.

Thus, a semiresistive region that does not greatly impair viewing, whichin any case is the aim with any semiresistive region referred to here,is used.

Unlike in document DE 1 256 812 B1, there are no overlapping elements ofopposite polarity in the region of the main viewing field and thecurrent flows, after activating the electrical power supply,approximately in a direction normal to the busbars and therefore in adirection parallel to the overall longitudinal direction of the blindlyterminating conducting strands. It should be understood that this“overall longitudinal direction” is the general direction or extensionalong which said strands extend from the busbars toward the main viewingfield.

Furthermore, the resistance of the transition between the busbar and thecoating is further reduced by greatly increasing the areas of contactcompared with the prior art. Consequently, the voltage needed to makethe heating currents flow over the heating surface is lower.

Admittedly, this configuration is preferably used for windshields inwhich good visibility in the central viewing field suffices for safedriving, however heated glazing units according to the invention mayalso be fitted at other places on the vehicle, and also in othermachines and moving equipment and in buildings.

Whereas in conventional solar cells with gate or comb electrodes thevoltage is applied over the thickness of the layer of the absorber, avoltage is applied in the application according to the invention for thepurpose of making a current flow in the plane of the coating. Theconducting strands and the semiresistive regions according to theinvention thus have the effect of bringing the busbars, customarilyplaced along the edge of the glazing, electrically closer togetherwithout however appreciably degrading the general viewing field of theglazing and without degrading at all the main viewing field A.

Moreover, it should be recognized that in document WO 00/72635 citedabove, the electric field, and consequently the heating field, does notinclude a semiresistive region since the strip that surrounds thecommunication window is excluded from the electric field owing to thefact that a resistive region is interposed between the closest busbarand the communication window that it surrounds. The trace of theelectric field lines in the last figure of the above document shows thatthe lines go around the communication window while still remaining inthe resistive coating, without passing through the strip that surroundsthe communication window.

When used in a vehicle, the configuration according to the inventionmakes it possible in particular for the windshield to be supplieddirectly, in order to heat it, with the usual onboard voltage of 12 to14 V DC, for which voltage a coating having as low as possible an ohmicresistance is of course recommended. The extent of the semiresistiveregions or else the length of the strands is dimensioned according tothe effective surface resistance of the coating chosen—the moreconducting the coating itself, the narrower the semiresistive regionsmay be or the shorter the strands may be.

Measured from the busbars, the extent of the semiresistive regions orthe length of the strands is greater than the width of the respectivebusbar to which they are attached and the strands that extend into theheating field in contact with the heating coating.

Likewise, with this configuration it is possible to keep the entirecoating on the surface of the transparent glazing—apart from theoptionally provided communication windows—in such a way that neithermasking nor removal of coating is necessary. Thus, the positiveproperties of the coating, namely in particular its infrared reflection(thermal insulation) and its uniform color (in reflection and intransmission), are preserved over the entire surface.

The conducting strands have a width and/or a thickness of preferably 0.5mm or less, and even more preferably 0.3 mm or less, measured inprojection on the surface of the glazing unit.

The additional conducting strands, which are also as thin as possible,only imperceptibly impede vision through the glazing.

Given that the transparent glazing is in almost all cases a laminatedglazing unit in which the coating itself is placed on a face lying onthe inside of the composite glazing, the conducting strands could also,apart from printing, be produced in the form of thin wires that arefixed, for example in a manner known per se, to a composite adhesivefilm and then deposited, with this film, on the coating, thus cominginto electrical contact with the coating. This contact is stable over along period after the final bonding of the laminated glazing.

In the embodiment in the form of screen-printed structures, theconducting strands are preferably deposited on a substrate (made ofglass or plastic or a plastic film) before the coating is applied. Thismay be carried out in a single operation together with the deposition ofthe actual busbars.

It is also possible to straddle, with a region of low resistance or withconducting strands, one or more communication windows produced along theedge of the glazing in the coating, without any risk of forming hotspots. The currents in the known problem areas along the lateral edgesof such communication windows are very greatly reduced by the strands.

Other details and advantages of the subject of the invention will becomeapparent from the drawings of an exemplary embodiment in the form of avehicle windshield and from their detailed description that follows.

In these schematic drawings, drawn to no particular scale:

FIG. 1 illustrates an embodiment of a transparent glazing unit with aresistive heating coating, in which embodiment busbars in the form ofstrips are connected to grid strands that extend in the form of veryfine fingers in the surface of the glazing;

FIG. 2 shows a second embodiment, in which the heating coating isdivided into current paths using separating lines;

FIG. 3 shows a partial cross section through a glazing unit according tothe invention along the line III-III in FIG. 1;

FIG. 4 shows a detail taken from FIG. 3;

FIG. 5 shows a detail similar to that of FIG. 4 for another embodimentdifferent from that of FIG. 4;

FIG. 6 illustrates another embodiment of a transparent glazing unit witha resistive heating coating, in which the heating coating includes atleast one semiresistive region; and

FIG. 7 illustrates another embodiment of a transparent glazing unit witha resistive heating coating, in which the adhesive layer has asemiresistive region, the figure being a partial cross section, similarto that illustrated in FIG. 3, through another glazing unit.

In the figures, an electrically resistive transparent coating 2 isplaced over the entire surface in a manner known per se in a heatedlaminated glazing unit 1 having an essentially trapezoidal (curvilinear)outline. The glazing unit 1 has been shown here only in one half—itsother half is equivalent.

The coating 2 is deposited in a known manner on a main face of asubstrate 11, this substrate then being integrated into the glazing unit1.

A broken line denoted by 20 indicates that the outer edge of thecontinuously coated surface lies all around, but slightly set backtoward the inside of, the peripheral outer edge of the laminated glazingunit 1, that is to say an edge band is provided in the coating allaround the surface. Thus, the coating is, on the one hand, electricallyisolated from the outside and, on the other hand, protected against anycorrosion damage penetrating via the outer edge of the glazing. Theouter edge 20 may be set back by removing the coating along the edge ofthe glazing, by masking the outline of the substrate before the coatingis deposited on this substrate, or else by tracing a separating linethat passes through the coating and runs along the outer edge of thesubstrate, which may be sufficient for meeting the isolation andcorrosion protection objectives.

The coating 2 itself is preferably made up, in a manner known per se,from a multilayer solar-protection system of high thermal resistancecomprising at least one metal functional layer and preferably at leasttwo metal functional layers, this system withstanding, without anydamage, the temperatures above 650° C. that are required for bending theglass panes, that is to say without its optical, electrical andheat-reflecting properties being degraded. The multilayer system alsoincludes, apart from the metal layers (which are preferably based onsilver), other layers such as antireflection layers and, optionally,barrier layers.

However, in relation to the present invention, it is also possible touse other electrically conducting multilayer systems that have a lowtemperature resistance, and in particular also multilayer systems thatare not deposited directly on a rigid glass pane but on a plastic film,(preferably a PET film). All these multilayer systems are preferablydeposited by sputtering (magnetron cathode sputtering).

The surface resistivity of the current multilayer systems of the typementioned above lies between about 0.5 and 5Ω/□. Vehicle windshieldswith such multilayer systems must achieve overall a light transmissionof at least 75% according to some standards, or 70% according to otherstandards.

Of course, the composition and the production of the coating are ofsecondary importance here, so that there is no need to dwell on detailsthereof.

An opaque colored layer 3 in the form of a peripheral frame has beendeposited along the edge of the laminated glazing unit 1, the inner edge30 of which layer, relative to the outer edge of the glazing unit,circumscribes the general viewing field of the transparent glazing unit1. This layer may lie in a plane of the laminated glazing unit otherthan that of the coating 2 (being located on the inside or on theoutside of the composite glazing unit). It serves as a layer forprotecting a bead of adhesive, with which the finished glazing unit isbonded to a vehicle body, from UV radiation. Moreover, it can concealfrom view connection elements for the main electrical heating functionand for the optional additional electrical functions of the glazing unit1.

Thus, the figure shows, along the upper edge of the laminated glazingunit 1, in the region of the surface covered by the colored layer 3, afirst busbar 4 and, along the lower edge, a second busbar 5. The twobusbars 4 and 5 are in direct conducting electrical connection with thecoating 2, in a manner known per se.

FIGS. 1, 2 and 5 show, in half in the middle of the glazing unit, acommunication window 22 below the busbar 4, which communication windowis also covered by the colored layer 3 and is therefore concealed fromview. It is also possible to provide several communication windows.

Many vehicle windshields are provided, along their upper edge, with aband (not shown here) that is bluish but transparent to light (“bandfilter”), which in particular reduces dazzling by sunshine. Likewise,such a band may also help to conceal the communication window from view.It may also replace part of the width of the band of the colored layer 3along the upper edge of the glazing unit, or it may be provided as acomplement thereto. Since the general viewing field of the glazing isdefined by the inner edge of the colored layer, it may consequentlyincorporate this bluish band.

As a general rule, the laminated glazing unit 1 is made up from tworigid glass and/or plastic panes 11 and 12 and from an adhesive layer 13joining the panes at the surface. The busbars 4 and 5 are placed on theadhesive layer 13 (for example a thermoplastic adhesive film made ofpolyvinyl butyral (PVB), ethylene/vinyl acetate (EVA) or polyurethane(PU)) and are fastened to its surface before the adhesive layer isassembled and bonded to the rigid panes.

The busbars 4 and 5 may be made up from thin narrow strips of metal(copper or aluminum) film, which are usually fixed beforehand to theadhesive film 13 and are applied together with an electrical contact tothe coating 2 during assembly of the laminated layers. However, theelectrical contact may also be provided by soldering the busbars 4 and5. During the subsequent autoclave process, a reliable contact is madebetween the busbars and the coating by the action of heat and pressure.

The busbars 4 and 5 may, as indicated above, be produced, as a variantor as a complement, by printing a conductive paste, which is baked whilethe glazing units are being bent. This is also appreciably lessexpensive than positioning parts of metal strips. In all cases, theprinted busbars have, during continuous manufacture, a higher ohmicresistance than that of the strips of metal film. The choice, betweenbusbars made of metal film and busbars in screen-printed band form,therefore depends only on the type of glazing and possibly on theoverall resistance of the multilayer heating system.

Compared with the coating 2, the busbars are always of negligible ohmicresistance and do not heat up appreciably during operation of theheating means.

Applying a voltage between the two busbars 4 and 5 in the heatingcoating generates an electric field and, through the resistive effect, aheating field.

Two (or more) heating fields may be provided, in a manner known per se,in the laminated glazing unit 1, which heating fields are to be suppliedseparately (for example with a vertical division down the middle of theglazing unit) and must also, of course, be connected to their respectivepower supply via separate conducting connectors. In this case, a commonground conductor may be used for the two heating fields, in such a waythat only the busbar 4 or the busbar 5 has to be divided into two parts,whereas the other busbar is continuous. In the first version, fourexternal connectors are required, whereas in the second only three arerequired.

The external connectors will not be discussed in detail here, becausethey have already been described many times in the art.

The outer boundary of the main viewing field A of the windshield, lyingon the inside of the general viewing field circumscribed by the edge 30of the colored layer 3, is indicated schematically by a dotted line L.The line L does not constitute an actual edge or the like in the glazingor in the coating, rather it serves merely to visually illustrate theapproximate position of the main viewing field A. The latter isdescribed in Annex 18 of the ECE R43 by means of certain parameters ofan arbitrary vehicle environment. In this field, no perceptibledegradation of vision having a size of greater than 30 microns ispermitted. In the secondary viewing field B, around the outside of themain viewing field A, slight limitations in vision, due to additions,etc., are permitted.

According to the invention, when an electrical supply voltage is appliedbetween the busbars 4 and 5, a current flows, forming a heating field inthe coating, this heating field including a semiresistive region 6 indirect contact with the upper busbar 4.

Thanks to the present invention, the heating current flows between thebusbars via the coating 2 in a heating field that has a region of lowerresistivity and then a region having a higher resistivity.

However, in a preferred version of the invention, the heating currentflows between the busbars via the coating 2 in a heating field that hasa region of lower resistivity and then a region of higher resistivityand then once again a region of lower resistivity.

As illustrated in FIGS. 1 and 6 in particular, an upper semiresistiveregion 6 thus extends downward beyond the area covered by the coloredlayer 3 in the general viewing field of the glazing toward the mainviewing field A and a lower semiresistive region 6′ extends upwardbeyond the area covered by the colored layer 3 in the general viewingfield of the glazing toward the main viewing field A.

In a first version of the invention, illustrated in FIGS. 1 to 5,starting from the upper busbar 4, a semiresistive region 6 formed by aset of lines extends beneath the colored layer 3, then, in the generalviewing field of the laminated glazing unit 1, from the edge regioncovered by the colored layer 3, into the secondary viewing field B.

These lines terminate blindly in the secondary viewing field B, more orless near the boundary of the main viewing field A. The length of thelines thus produced depends directly on the conductivity of the coatingchosen.

These lines represent conducting strands 46, which are electricallyconnected to the busbar 4 and to the coating 2 and have a low ohmicresistance compared with the latter. A number of these lines alsostraddle the communication window 22 in such a way as to guarantee thatthe coating 2 on either side of the communication window, seen from thebusbar 4, is supplied directly with power. From the visual standpoint,these lines that straddle the communication window 22 are masked by thecolored layer 3. Another form of masking may, as already indicated, beoptionally provided by a band of bluish color (band filter), but notshown here.

Conducting strands 56, similar to the conducting strands 46, also extendinto the viewing field B of the laminated glazing unit 1 from the lowerbusbar 5.

In each case, the combination of the conducting strands, 46 or 56respectively, forms with the coating 2 a semiresistive region accordingto the invention.

It is unnecessary to provide such conducting strands 46 and 56 for bothbusbars, 4 and 5 respectively. However, if conducting strands areprovided on both sides of the main viewing field A, they in no caseextend in such a way that conducting strands of opposite polarityoverlap in a projection perpendicular to their overall directions. Thus,the central part of the general viewing field and of the heating field,and in particular the main viewing field A, remains undisturbed.

Whereas in conventional glazing units with layer heating without asemiresistive region the heating current has to flow only via thecoating over the entire distance between the busbars, this distance maybe reduced to values of between 25% and 80% by the semiresistiveregion(s) according to the invention and in particular by the conductingstrands according to the present invention, depending on the extent ofthe viewing field A, some of the current spanning the remaining distancein the semiresistive region(s) and in particular in the conductingstrands.

In FIGS. 1 and 2, the strands 46 and 56 are placed uniformly apart andare all produced with the same length.

The inner end of the conducting strands is substantially at the samepotential as the busbars to which they are connected.

In the general viewing field of the glazing unit, the current flowsapproximately perpendicular to the busbars 4 and 5 and parallel to thelongitudinal general direction of the conducting strands 46, 56.

Thus, the current flows in the general viewing field in a directionapproximately parallel to the longitudinal direction of the conductingstrands.

As previously, a flow of current, even though small, remains over theentire surface of the coating, because the busbars are not separatedfrom the coating in the parts located between the conducting strands.However, this current flow does not result in the formation of hot spotsalong the edges of the communication window 22.

To meet the abovementioned objectives of the present invention, theconducting strands, 46 and 56 respectively, must also be in intimategalvanic contact with the coating, in addition to their goodconductivity (so as thereby to form, as mentioned above, a semiresistiveregion). Certainly, in principle it is conceivable to produce them inthe form of wire portions. However, they are preferably printed, beforethe coating is deposited, on that surface of the glazing unit thatsubsequently will have to bear the coating. Certainly it is alsopossible to print them on the finished coating, but this runs the riskof damaging the multilayer coating, which is mechanically weak.

Preferably, the conducting strands are printed using a very conductivescreen-printing paste containing at least 80%, and preferably more than85%, silver.

These conducting strands preferably have a dark color seen via an outerface of the glazing, so as to be not easily perceptible to the viewer'seyes when he is looking from the outside into the vehicle and, alsopreferably, have a light color, seen via an inner face of the glazing,so as to be not easily perceptible to the eyes of the viewer when he islooking out from inside the vehicle.

If printed busbars 4 and 5 are used, they may then be produced in asingle operation at the same time as the conducting strands 46/56 andusing the same screen-printing paste. No separate operation is thenneeded to bring the conducting strands into electrical contact with thebusbars.

However, if busbars in the form of strips of metal film are used, thesethen have to be connected with a low ohmic resistance to the coating andto the conducting strands. The strips of film, which are preferablytinned, are therefore soldered to the conducting strands in a mannerknown per se. In principle, the screen-printing pastes with a high metalcontent that are used here are easily soldered to the tinned metalstrips.

The lengths, separations and number of the conducting strands 46 and 56and the dimensions of the busbars can only be shown here schematically.However, their relative dimensions may be seen—whereas the busbars 4 and5 are produced in the form of a usual band with a width of a fewmillimeters, the conducting strands 46 and 56 are as close together andas invisible as possible, and however appreciably longer than the widthsof the busbars.

The individual configuration in a specific laminated glazing unit maycertainly be broadly predetermined by simulation. However, it willdepend again very greatly on the respective magnitude of the dimensionsof the specific glazing, on the type of busbars and on the actualelectrical properties of the coating. It may for example also besufficient to add conducting strands only to one of the busbars. For arelatively short distance between the two busbars 4 and 5, theconducting strands themselves may also be shortened.

In the case of straight lines for a vehicle glazing unit, these linesare not parallel but converge toward the busbar to which they areattached, preferably in such a way that a longitudinal symmetry (alongthe longitudinal axis of the vehicle) may be observed in the glazing.

It has been determined, for one specific type of glazing unit, thatspacing of 25 mm between the various conducting strands can be used.However, the surface heating power levels available in the semiresistiveregions can if necessary be adjusted, by varying the spacing, for agiven resistance of the conducting strands. Furthermore, to simplifymatters, only straight conducting strands have been shown here, but thisdoes not exclude the possibility in practice of producing them in curvedand/or corrugated forms, and/or in the form of open or closed loopsand/or in portions of arcs and/or in meanders, which could be lessvisible.

FIG. 2 shows an alternative embodiment in which the coating 2 is dividedby separating lines 24 in the general viewing field. The separatinglines 24 may pass entirely through the coating down to the surface ofthe substrate, or else they may penetrate only as far as the conductingpartial layer close to the substrate. They have to divide the coating,which by nature is continuous, into current paths. Various techniquesexist for producing such separating lines, among which laser cutting iscurrently the most common, because it is the most economic regarding theresult. In particular, the separating lines that can be produced arethus extremely narrow and only perceptible to the naked eye withdifficulty.

If FIG. 2 is taken as representing the view seen by the driver of aleft-hand drive vehicle, he will usually have to look through thesurface part in which the separating lines 24 are the closest together.The purpose of these lines is to collect the current flow through thecoating 2 in the main viewing field A precisely in this region, and thusprovide the highest heating power in this main viewing region when hisvision is obstructed by snow, ice or water droplets and to provide clearvision as quickly and as effectively as possible.

Here too, the arrangement of the separating lines 24 has been indicatedonly schematically and only few conclusions may be drawn as to actualconfigurations. It is also not always judicious to always tracecontinuous separating lines, rather it is possible to produce some orall of them as segmented separating lines, so to speak dotted lines, orto provide, instead of the longer separating lines, a few short portionsin order to deflect the current in certain predetermined paths. However,this is already also known from document DE 36 44 297 A1 mentionedabove.

However, here again, it is obvious that the heating current in thegeneral viewing field flows approximately in an overall directionperpendicular to the busbars and parallel to the longitudinal axes ofthe conducting strands 46/56.

FIG. 3 shows a sectional view through the edge of the glazing unit 1along the line III-III of FIG. 1. Two rigid individual panes 11, 12(made of glass or plastic) and an electrically insulating adhesive layer13, which is optically transparent and joining said panes together bybonding in the usual manner, may be seen. This adhesive layer 13 issubdivided horizontally by dot-dash lines in order to indicate that itis in fact considerably thicker than the transparent coating 2 depositedon the lower pane 12. This coating is shaded gray here for the sake ofvisibility. The adhesive layer may be formed in the usual manner by aPVB film having a thickness of about 0.76 mm.

The numerical references in FIGS. 1 and 2 have been preserved. Thecoating 2, the outer edge region of which is separated by the separatinglines 20, is, as may be seen, located on the pane 12 above the busbar 5and the conducting strands 56 connected to the latter, which strandshave been deposited here, before deposition of the coating 2, in theform of screen-printed structures. The opaque colored layer 3 here isprinted on that face of the pane 11 located on the inside of thecomposite glazing unit and overlaps, in vertical projection (the viewingdirection) the separating line 20, the busbar 5 and the conductingstrands in their part which is connected directly to the busbar.However, the conducting strands also extend beyond the edge 30 of theopaque colored layer 3 as far as the general viewing field of theglazing unit 1.

The opaque colored layer 3 could also, unlike in the representation, belocated on the outer faces (not visible here) of one of the panes 11 or12, or also on the same surface as the coating 2 and the busbars 4 and5.

FIG. 4 illustrates the arrangement of the conducting strands throughanother section, the viewing direction of which starts from the right inFIG. 3 in the plane of the surface of the pane 12. The view is thereforetoward the front faces of the conducting strands 56, in the backgroundof which the busbar 5 extends transversely to the viewing direction.

To improve viewing, in the region of the central conducting strand inFIG. 4, the coating 2 on the surface of the pane 12 has been partlyremoved. More precisely, it may be seen that both the busbar and theconducting strand lie beneath the coating 2 on the surface of the pane12.

In another embodiment, illustrated in FIG. 5, with the busbarmanufactured from strips of metal films, these would be applied to thecoating 2 and assembled to the latter as continuously as possible andwith as low a resistance as possible by soldered assemblies or, wherenecessary, also by conductive adhesives (this also being known as analternative). The soldered assemblies must of course be provided inparticular at the conducting strands.

In a second version of the invention, illustrated in FIG. 6, startingfrom the upper busbar 4, a semiresistive region 6 formed by a particularregion of the heating coating 2 and illustrated by crossed dotted lines,extends beneath the colored layer 3 and then into the general viewingfield of the laminated glazing unit 1 from the edge region covered bythe colored layer 3 toward the inside of the secondary viewing field B.In this semiresistive region 6, the resistance of the heating coating 2is less than the resistance of the heating coating outside anysemiresistive region.

In this semiresistive region 6, the resistance of the heating coating 2is two times, five times or even ten times less, or even less, than theresistance of the resistive coating outside any semiresistive region.

Starting from the lower busbar 5, a semiresistive region 6′ formed byone particular region of the heating coating 2, similar to thesemiresistive region 6 and illustrated also by crossed dotted lines,extends into the viewing field B of the laminated glazing unit 1.

It is not necessary to provide such semiresistive regions of the coatingfor both busbars, 4 and 5 respectively. However, if semiresistiveregions of the coating are provided on both sides of the main viewingfield A, they do not extend into the central part of the general viewingfield and of the heating field, and in particular the main viewing fieldA remains undisturbed.

This second version may be obtained in industrial practice by depositingadditional layers in semiresistive regions, either during deposition ofthe heating coating itself, by transverse variations in the thicknessesof the layers on the large plates coated continuously with the heatingcoating, or after the substrates coated with the heating coating havebeen cut out, by an additional local deposition on these substrates.

In a third version of the invention, illustrated in FIG. 7, the glazingis a composite glazing unit that includes at least one adhesive layer13′ in electrical contact with the heating coating 2. This adhesivelayer 13′ includes at least one semiresistive region illustrated here bya band 60 of semiresistive plastic. This band is included in that faceof the adhesive layer that will be in contact with the heating coating 2during manufacture of the layer so that, during manufacture of theglazing unit, the plastic band 60 is in contact with the busbar 5.

This third version is illustrated in the lower part of the glazing unit,but it may of course also, or instead, be used in the upper part of theglazing unit.

It is also possible for the plastic band or portion to be produced overthe entire thickness of the adhesive layer 13′ or else for it to beformed by a strip of plastic that incorporates a conducting grid on itssurface in contact with the heating coating.

It is also conceivable to combine the plastic band 60, which is possiblyconducting, with the coating 2 as a semiresistive region, since in eachcase that part of the coating 2 which is covered by said band 60conducts at least part of the heating current.

To produce such a semiresistive or quite conductive plastic band 60, itis for example possible to dope a “matrix” material of the thermoplasticfilm used (PVB, EVA or polyurethane) with conducting particles,especially metal particles, of such a density that the region or volumein question is at least semiresistive or even conductive. Such doping ispossible without excessively impeding the light transmission.

The present invention has been described in the foregoing by way ofexample. Of course, a person skilled in the art is capable of producingvarious alternative embodiments of the invention without therebydeparting from the scope of the patent as defined by the claims.

A person skilled in the art is in particular capable of combiningvarious versions and embodiments of the invention described above.

1-29. (canceled)
 30. A transparent glazing unit comprising: a resistiveheating coating that extends over a part of the glazing unit, or over amain viewing field, and that is electrically connected to at least twobusbars such that, when an electrical supply voltage is applied betweenthe busbars, a current flows, which heats a heating field in thecoating, wherein the heating field includes at least one semiresistiveregion in direct contact with at least one of the busbars.
 31. Theglazing unit as claimed in claim 30, wherein the main viewing field doesnot include a semiresistive region.
 32. The glazing unit as claimed inclaim 30, wherein at least one semiresistive region is in direct contactwith at least one of the busbars at a positive potential.
 33. Theglazing unit as claimed in claim 32, wherein at least one semiresistiveregion is in direct contact with at least one of the busbars at anegative potential.
 34. The glazing unit as claimed in claim 33, whereinthe main viewing field lies between at least two semiresistive regions.35. The glazing unit as claimed in claim 30, wherein at least one partof a peripheral edge of the glazing unit is concealed by an opaquecolored layer, or a part in a region of the busbars, at least onesemiresistive region lying in the main viewing field of the glazing,beyond an area covered by the colored layer.
 36. The glazing unit asclaimed in claim 30, wherein resistance of the heating coating in thesemiresistive region is at least less than resistance of the heatingcoating outside any semiresistive region.
 37. The glazing unit asclaimed in claim 36, wherein the resistance of the heating coating inthe semiresistive region is at least two times, five times, ten timesless, or even less, than the resistance of the resistive coating outsideany semiresistive region.
 38. The glazing unit as claimed in claim 30,as a composite glazing unit that includes at least one adhesive layer inelectrical contact with the heating coating, the adhesive layerincluding the at least one semiresistive region.
 39. The glazing unit asclaimed in claim 38, wherein the semiresistive region of the adhesivelayer includes at least one band of the adhesive layer doped withconducting particles.
 40. The glazing unit as claimed in claim 30,wherein the semiresistive region includes conducting strands formed fromconducting printed lines and/or conducting wires.
 41. The glazing unitas claimed in claim 40, wherein the strands start from at least one ofthe busbars, a length of the strands being greater than a width of therespective busbar, and the strands extending into the heating field incontact with the heating coating.
 42. The glazing unit as claimed inclaim 40, wherein the strands have a width and/or a thickness of 0.5 mmor less, or 0.3 mm or less, measured in projection on the surface of theglazing unit.
 43. The glazing unit as claimed in claim 40, wherein thestrands formed from conducting printed lines are printed on the heatingcoating.
 44. The glazing unit as claimed in claim 40, wherein thestrands formed from conducting wires are electrically connected to theheating coating and at least to the semiresistive region by soldering atleast at discrete contact points.
 45. The glazing unit as claimed inclaim 40, wherein the current flows in the main viewing field in adirection approximately parallel to the longitudinal direction of theconducting strands.
 46. The glazing unit as claimed in claim 40, whereinthe strands are dark in color when viewed from an external face of theglazing unit.
 47. The glazing unit as claimed in claim 40, wherein thestrands are light in color when viewed from an internal face of theglazing unit.
 48. The glazing unit as claimed in claim 40, wherein thestrands are placed uniformly apart.
 49. The glazing unit as claimed inclaim 40, wherein the strands are all produced with a same length. 50.The glazing unit as claimed in claim 40, wherein the strands areproduced in a form of straight lines, open or closed loops, portions ofarcs, and/or meanders.
 51. The glazing unit as claimed in claim 30,wherein the heating coating is divided by separating lines in the mainviewing field, which lines divide up the heating coating into currentpaths.
 52. The glazing unit as claimed in claim 51, wherein theseparating lines collect current in the main viewing field.
 53. Theglazing unit as claimed in claim 30, wherein the busbars are produced byprinting and/or from metal strips.
 54. The glazing unit as claimed inclaim 53, wherein the printed busbars are printed on the heatingcoating.
 55. The glazing unit as claimed in claim 54, wherein thebusbars produced from metal strips are electrically connected to theheating coating and to the semiresistive region by soldering at least atdiscrete contact points.
 56. The glazing unit as claimed in claim 30,wherein the semiresistive region is placed only over part of thelongitudinal extent of one or each busbar.
 57. The glazing unit asclaimed in claim 30 in a form of a vehicle windshield, wherein thesemiresistive region extends at most as far as a boundary of anormalized viewing field of the windshield.
 58. The glazing unit asclaimed in claim 30, wherein a bluish band extends at least along itsupper edge and at least partly conceals the semiresistive regionpositioned at an upper edge.